NADH dehydrogenase (NADH:ubiquinone oxidoreductase, NADH-D) is the first multienzyme complex (Complex I) in a chain of three complexes that make up the mitochondrial electron transport chain. The mitochondrial electron transport chain is responsible for the transport of electrons from NADH to oxygen and the coupling of this oxidation to the synthesis of ATP (oxidative phosphorylation) which provides the energy source for driving a cell's many energy-requiring reactions. NADH-D accomplishes the first step in this process by accepting electrons from NADH and passing them through a flavin molecule to ubiquinone, which transfers the electrons to the second enzyme complex in the chain.
NADH-D and the other members of the electron transport chain are located in the mitochondrial membrane. NADH-D is the largest of the three complexes with an estimated mass of 800 kDa comprising some 40 polypeptide subunits of widely varying size and composition. The polypeptide composition of NADH-D in a variety of mammalian species including rat, rabbit, cow, and man is very similar (Cleeter, M. W. J. and Ragan, C. I. (1985) Biochem. J. 230: 739-46). The best characterized NADH-D is from bovine heart mitochondria and is composed of 41 polypeptides (Walker, J. E. et al. (1992) J. Mol. Biol. 226: 1051-72). Seven of these polypeptides are encoded by mitochondrial DNA while the remaining 34 are nuclear gene products that are imported into the mitochondria. Many of these imported polypeptides are characterized by various N-terminal peptide sequences or modified N-terminal amino acids (myristoylation or acetylation) that target them to the mitochondria and are then cleaved from the mature protein. However, several of these polypeptides have neither N-terminal targeting sequences nor modified-N terminal amino acids. Their import signals appear to lie within the mature protein (Walker et al., supra).
The functions of many of the individual subunits in NADH-D are largely unknown. The 24-, 51-, and 75-kDa subunits have been identified as being catalytically important in electron transport, with the 51-kDa subunit forming part of the NADH binding site and containing the flavin moiety that is the initial electron acceptor (Ali, S. T. et al. (1993) Genomics 18:435-39). The location of other functionally important groups, such as the electron-carrying iron sulfate centers, remains to be determined. Many of the smaller subunits (&lt;30k Da) contain hydrophobic sequences that may be folded into membrane spanning .alpha.-helices. These subunits presumably are anchored into the inner membrane of the mitochondria and interact via more hydrophilic parts of their sequence with globular proteins in the large extrinsic domain of NADH-D.
Defects and altered expression of NADH-D are associated with a variety of disease conditions in man, including neurodegenerative diseases, myopathies, and cancer. In addition, NADH-D reduction of the quinone moiety in chemotherapeutic agents such as doxorubicin is believed to contribute to the antitumor activity and/or mutagenicity of these drugs.
The discovery of new NADH dehydrogenase subunits and the polynucleotides encoding them provides a means to investigate mitochondrial respiratory mechanisms under normal and disease conditions and satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the treatment or prevention of cancer and immune disorders.